It's a bug in NodeXL. The ReservedMetadataKeys.PerColor key can have either a System.Drawing.Color or a System.Windows.Media.Color value, but there is a place in NodeXL's "collapse group" code that mistakenly checks only for a System.Drawing.Color.

This will be fixed in the next release, which will be version 1.0.1.231. In the meantime, you can work around the bug by using System.Drawing.Color values for vertex colors instead of System.Windows.Media.Color values. You can convert a System.Windows.Media.Color
to a System.Drawing.Color like this: